Compressors and especially piston compressors are standard for the compressing of liquids or gases. In order to minimize the friction forces occurring between the moving parts of the compressors, compressors with oil lubrication are preferred. This oil lubrication has the task of providing a preferably hydrodynamic tribological contact between the sliding parts at the piston and guide rings and at the seals of the piston rod. Thanks to this tribological contact, very low rates of wear can be achieved for these sealing elements. Thus, standard lifetimes for lubricated machines of over 25000 hours with no significant wear can be achieved.
However, the oil lubrication comes with the risk of lubricant getting dissolved in the gases or liquids being sealed. Consequently, oil-lubricated compressors are unsuitable for sensitive media such as are used, for example, in the food industry or in the medical field.
To overcome this problem, piston compressors are being used increasingly with piston rod seals having no oil lubrication. This has been made possible by the development of sealing elements based on plastics.
Such sealing elements are described, for example, in DE 10 2006 015 327 B9. As the materials for piston rod sealing rings, plastics such as ballasted polymers have chiefly worked well. One often used polymer material is polytetrafluorethylene, for example. Solids such as amorphous carbon, graphite, glass fibers, metals, ceramics or solid lubricants are incorporated into the PTFE matrix. To increase the service life, usually several piston rod sealing rings, at least two, are arranged one behind the other in the axial direction and form a sealing element set, also known as a seal packing.
However, without the oil lubrication being present there are great changes in the tribological properties at the contact sites of the sliding parts. The hydrodynamic tribological contact becomes a tribochemical contact, which only results in good sliding performance and low rates of wear if a so-called transfer film is formed. Thanks to mechanical/physical forces between the sliding parts, structural changes occur in the surface of the sliding layer. These can be surface increases, decrease in particle size, formation of fresh surfaces, material abrasion, or even sometimes phase transformations, which are generally subsumed under the terms tribochemical contact or tribochemical process. However, this transfer film must be constantly renewed by additional tribochemical processes. Once a stationary renewal process has been established, low friction values and rates of wear are possible, but they are still substantially higher than those of the oil-lubricated sliding parts. Usually only around 8000 hours of operation can be achieved today in dry running conditions.
Due to the heightened friction values of the oil-free sliding part, the movement of the piston rod produces an increased output of frictional heat and thus an increased temperature at the contact sites.
However, extensive tribological studies have shown that this increased temperature in turn has a negative impact on the rates of wear of the sliding parts. In the worst case, this can result in premature failure of the compressor. Thus, an effective cooling of the sliding parts constitutes a major problem with dry lubrication.
A good cooling must be provided at the piston and guide rings, since the cylinder bushing and thus also the contact site between piston and cylinder bushing can be cooled, but not in the case of a piston rod seal.
The sealing rings of the piston rod seal are arranged in the so-called seal packing, also known as a packing gland. The chambers of the seal packings are usually filled with water. However, the cooling is not very effective, since a process gas present between the contact surface and the chambers prevents a good heat flow.
The bulk of the frictional heat produced is transported by thermal conduction along the piston rod from the region of the seal packing to a region at a distance from the seal packing. Here, the heat is ultimately taken away to the surroundings by the forced convection of the moving piston rod.
Yet conventional piston rods consist of steel materials, for example, and thus they have only slight thermal conductivities (steel: 15-58 W/(m·K)). This low thermal conductivity necessarily results in a large temperature gradient from the seal packing region (high temperature) to the region away from the seal packing (low temperature).
However, actively cooled piston rods are known from the prior art for an improved cooling of the piston rod.
Thus, for example, patent DE PS 340 086 discloses a piston rod for dual-action internal combustion engines, having a central borehole and a number of boreholes situated in proximity to the surface of the rod, so that the surface can be cooled by a coolant flowing through the boreholes.
But this device has the drawback that ports for the flowing coolant need to be provided at the piston rod. Furthermore, a circulating pump needs to be in constant operation, pumping the coolant through the piston rod. Both the ports and the pump increase the cost and maintenance expense of such cooled piston rods. Moreover, an unnoticed failure of the circulating pump results in an immediate rise in temperature of the piston rod and thus concomitant damage to it. Therefore, the functionality of the circulating pump must be constantly monitored, which likewise entails increased cost and time expense.
From DE 199 01 868 B4 there is known a piston rod having at least one coolant supply channel and at least one coolant drain channel. In addition, the piston rod has an axial blind borehole, and the at least one coolant supply channel and the at least one coolant drain channel are each arranged at the side of this blind borehole.
In addition to a cooling by the coolant channels, the blind borehole provides a weight reduction, so that during horizontal operation of the piston rod there should be reduced friction and thus less wear and tear.
But since the piston rod of DE 199 01 868 B4 is likewise cooled actively by means of coolant, the same drawbacks occur as were discussed in connection with the patent DE PS 340 086.
Liquid-cooled piston rods are also known from CH 163 967 and DE 521 491, which accomplish a temperature decrease for the piston rod, but likewise have the drawbacks of the patent DE PS 340 086.